There's one possible issue, here: if iOS were to become
more like macOS (allowing installing and execution of apps from anywhere), then who says these apps have to follow app store guidelines, such as utilizing sandboxes?
From
Apple's guidelines for making applications that can be distributed on the mac App Store:
As it has been pointed out, iOS was originally forked from macOS, and has the same underlying POSIX subsystem -- the forced opening up of iOS to allow unhindered sideloading might open up the ability to run apps that don't adhere to Apple's sandbox containers, as is the case on macOS.